Hi,
I am looking at a missed optimization and I think this is something that
could be added to compare-elim, if it's not already done somewhere else.
I have a double word comparison to zero, so in C it's:
int le(long a) { return a <= 0; }
My expand uses the following transformation (in my current discussion y0
and y1 are 0):
if x0:x1 <= y0:y1 goto L
==>
if x0 < y0 goto L
if x0 > y0 goto K
if x1 <= y1 goto L
K:
This generates the assembler:
$le:
tst @$XAP_AH
bmi ?L7
cmp AH,#H'0000
bgt ?L3
tst @$XAP_AL
bne ?L3
?L7:
ld AL,#H'0001
bra 0,X
?L3:
ld AL,#H'0000
bra 0,X
My argument is that the tst @$XAP_AH above could be transformed into cmp
AH, #H'0000 and the following compare could be removed getting this:
$le:
cmp AH,#H'0000
bmi ?L7
bgt ?L3
tst @$XAP_AL
bne ?L3
?L7:
ld AL,#H'0001
bra 0,X
?L3:
ld AL,#H'0000
bra 0,X
The RTL for the first two comparisons at compare-elim is:
(insn 51 3 52 2 (set (reg:CC_NZ 13 CC)
(compare:CC_NZ (reg:QI 0 AH [orig:27 a ] [27])
(const_int 0 [0]))) b651.c:1 63 {*tstqi}
(nil))
(jump_insn 52 51 34 2 (set (pc)
(if_then_else (ge (reg:CC_NZ 13 CC)
(const_int 0 [0]))
(label_ref 41)
(pc))) b651.c:1 68 {*conditional_branch}
(expr_list:REG_BR_PROB (const_int 7900 [0x1edc])
(nil))
-> 41)
(code_label 41 43 25 4 6 "" [1 uses])
(note 25 41 49 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
(insn 49 25 50 4 (set (reg:CC 13 CC)
(compare:CC (reg:QI 0 AH [orig:27 a ] [27])
(const_int 0 [0]))) b651.c:1 64 {*cmpqi}
(nil))
(jump_insn 50 49 26 4 (set (pc)
(if_then_else (gt (reg:CC 13 CC)
(const_int 0 [0]))
(label_ref 13)
(pc))) b651.c:1 68 {*conditional_branch}
(expr_list:REG_BR_PROB (const_int 7900 [0x1edc])
(nil))
-> 13)
The transformation would involve deleting insn 49 and transforming insn
51 into mode CC. I guess this could be easily done by checking at each
comparison if we can eliminate the current comparison by generalizing
the previous one using a function like rx_cc_modes_compatible (from rx.c).
What do you think?
Cheers,
--
PMatos
